build: lint "strtol" #40442

Problem:
No lint for strtol().

Solution:
Add lint, and update existing usages.

Callers that previously got a (garbage) large positive value:
- `indent.c`: use getdigits() (intmax, no int truncation) with def=1 so
  overflow/too-large values stay positive and fall through to the
  existing "too big" check (E475).
- `file_search.c`: use getdigits() with def=255 so overflow/too-large
  values keep the "max expand" (else) branch.

The other conversions (api/command.c, eval/window.c, highlight_group.c,
tui/tui.c) only diverge on pathological overflow inputs where strtol's
result was already garbage and the observable outcome is unchanged.

test/functional/script/clint_spec.lua

@@ -43,7 +43,8 @@ describe('clint.lua', function()
       'test/functional/fixtures/clint_test.c:138:  Use xfree(...) instead of free(...).  [runtime/memory_fn] [2]',
       'test/functional/fixtures/clint_test.c:141:  Use os_getenv(...) instead of getenv(...).  [runtime/os_fn] [2]',
       'test/functional/fixtures/clint_test.c:142:  Use os_setenv(...) instead of setenv(...).  [runtime/os_fn] [2]',
-      'Total errors found: 28',
+      'test/functional/fixtures/clint_test.c:143:  Use getdigits()/getdigits_int() (or vim_str2nr() for non-decimal bases) instead of strtol, which overflows silently. See src/nvim/charset.c.  [runtime/deprecated] [4]',
+      'Total errors found: 29',
     }
     t.eq(expected, output_lines)
   end)